An isothiocyanate group is a very useful functional group in synthetic organic chemistry since its reactivity is high and it can be led to various chemical structures. And, a carboxyl group is a useful functional group in the field of organic materials, drugs or agricultural chemicals because of its characteristic acidity and hydrogen bonding ability. Accordingly, an isothiocyanate compound having a carboxyl group(s), which has such two functional groups in one molecule, can be said to be a very useful compound as a product or a synthetic intermediate in the field of organic materials, drugs or agricultural chemicals. As an example, it is known that 3,5-diisocyanatobenzoic acid is useful as a starting material for the synthesis of a metal-binding polypeptide (e.g. Patent Document 1).
As a method for producing an isothiocyanate compound having a carboxyl group(s) from the corresponding amino compound having a carboxyl group(s), a method of using thiophosgene (e.g. Non-Patent Document 1) and a method of using chlorothionoformic acid phenyl ester (e.g. Non-Patent Document 2) have been reported, however, they have a problem that these compounds have a very strong toxicity and bad odor. Further, a method of using tetramethylthiuram disulfide has been reported (e.g. Non-Patent Document 3), but it has a problem that isolation of reaction intermediate is necessary, whereby the operation is cumbersome; the reaction condition is severe such that heating is carried out at a high temperature in the presence of acid; and tetramethylthiuram disulfide to be used is expensive. Therefore, it has been desired to develop a novel method for producing an isothiocyanate compound having a carboxyl group(s) in a high yield and with high purity without using such reagents, which is useful also as an industrial production method.